Just in the middle of finishing up an airbox for the PP. I'll have two 3" feeds running down to each headlight with filters in each one. Will this system be able to supply enough air to the engine?

 

Started on cold air boxes for the filters. I absolutely hate the cheap ebay lights but with one of the lights removed it makes for a perfect air scoop to feed the filters.

 

paul yaw did something like that back in the day, it seems like it should be totally fine.

in case you would like to do maths, Mazda specs a 110-120mm duct for a 12A, so you can add the % for a 13B, and then get area, and compute for dual instead of single

 

Here is a pic of the baseplate on our RX3. I have no pic of the airbox but in this movie you can see it quite well.
the airbox is about 140mm high and it fed by a 6" tube, the airfilter is a 6" cone. cold air gets in by leaving one of the headlights out.

shown power on the movie is incorrect due to wrong engine loss run. actual power was 272 hp but we had issues with carb emptying the float bowl.

your design should work also well but make sure there is enough space above the velocity stacks, especially if you want to change the length afterwards. we found that 20mm makes about 400 rpm difference in peak torque